Pain is a complex disorder with neurochemical and psychological components contributing to the severity, the persistence, and the difficulty in adequately treating the condition. Opioids and cannabinoids are two classes of analgesics that have been used to treat pain for centuries and are arguably the oldest of “pharmacological” interventions used by man. Unfortunately, they also produce several adverse side effects that can complicate pain management.
Opioids produce their pharmacological effects through activation of G protein-coupled receptors (GPCRs). There are four distinct genes coding for opioid receptors: the mu-, kappa-, and delta-opioid receptors (MOR, KOR, and DOR, respectively) and the opioid-like receptor1 [ORL-1 or the nociceptin receptor (NOP)] (Cox 2012; Pasternak 2013). The generation of genetic knockout mice has demonstrated that the majority of clinically used opioids including morphine produce their pharmacological effects primarily by activating the MOR (Matthes et al., 1996; Sora et al., 1997; Roy et al., 1998; Kieffer 1999; Kieffer and Gaveriaux-Ruff 2002). The MOR is widely distributed and expressed in neurons in the brain, spinal cord, and the periphery (Gutstein and Akil 2001).
The pharmacological and genetic studies of rodents to date suggests that the development of opioid agonists that bias MOR toward G protein signaling cascades and away from β-arrestin interactions may provide a novel mechanism by which to produce analgesia with less severe adverse effects. Presently, there have been a few reports of “biased” MOR agonists and their effects in vivo. One such compound herkinorin is a selective MOR agonist that does not recruit β-arrestin1 or β-arrestin2 in cell culture assays (Groer et al., 2007). In an inflammatory pain model in rat, herkinorin reduces formalin-induced flinching to the same degree as morphine when administered at the same dose (10 mg/kg, i.pl.), an effect that is reversed by the opioid antagonist naloxone (Lamb et al., 2012). Moreover, antinociceptive tolerance to herkinorin does not develop to repeated treatment over a 5-day period and it produces antinociception in morphine-tolerant rats (Lamb et al., 2012). The initial studies with these MOR “biased” agonists lend further support to the idea that developing a MOR agonist that does not engage β-arrestins but fully activates G protein signaling may provide a novel therapeutic avenue to improve pain treatment with opioids.
The binding of an activating ligand (agonist) to the extracellular side of a GPCR results in conformational changes that enable the receptor to activate heterotrimeric G proteins. Despite the importance of this process, only the β-adrenergic receptor, the M2 muscarinic receptor and rhodopsin have been crystallized and their structures solved in agonist-bound active-state conformations (Choe et al., 2011; Rasmussen et al., 2011a; Rasmussen et al., 2011b; Deupi et al., 2012; Scheerer et al., 2008; Kruse et al., 2013). Crystallization of agonist-bound active-state GPCRs has been extremely challenging due to their inherent conformational flexibility. Fluorescence and NMR experiments have shown that the conformational stabilization of the agonist-bound active-state conformation requires that the receptor must form a complex with an agonist and its G protein, or some other binding protein that stabilizes the active conformation (Yao et al., 2009; Nygaard et al., 2013).
The crystal structure of MOR available today will enable the application of structure-based approaches to design better drugs for the management of pain, addiction and other human diseases, where MORs play a key role. Though crystal structures were obtained for the μ-opioid receptor bound to an antagonist (Manglik et al., 2012), experimental data that reveal the agonist-bound active state opioid receptor protein structure have not been reported. Such information could greatly facilitate the development of novel agents, not only with increased potency and selectivity, but eventually also biased agonists.
The development of new straightforward tools for structural and pharmacological analysis of GPCR drug targets is therefore needed.